In some types of integrated circuits, such as in semiconductor devices, current flowing through a defect in the circuit may cause electron-hole recombination. Current propagated in this manner creates photons which can be detected with a photon receptor. The receptor can resolve an image of the current path using the photons that are produced by recombination along the path. The method of imaging a circuit in this manner is generally termed background emission photometry.
The ability to trace the path taken by electrical current in a circuit is very useful. For example, in a circuit that is not operating correctly, such a representation of the current path may be useful in determining the location of the defect in the circuit. A short could be detected in this manner.
Depending on the amount of recombination occurring, it may take several minutes to collect enough photons for the receptor to make a distinguishable image. Thus, this type of imaging works well for static circuits which can hold an electrical charge for periods of time in excess of that required to resolve the image. An example of such a circuit would be a static random access memory device, or SRAM.
Dynamic devices, such as DRAM, or dynamic random access memory, cannot hold a charge for such a length of time. Therefore, dynamic devices are difficult to image with background emission photometry because relatively few photons are generated during the short time that the device holds a charge. The number of photons generated is typically insufficient for the photometer to resolve an image.
During normal operation, dynamic devices are typically refreshed on an intermittent basis. Unfortunately, when a device is refreshed in this manner, the current that flows through the circuit during the refresh cycle produces an enormous level of background photon emission. This high level of emission would obscure the photon emission from a defect, which typically carries less current that those portions of the circuit which are designed to carry current. Thus, background emission photometry has been of limited value in diagnosing dynamic integrated circuits.
What is needed, therefore, is a method and apparatus that overcomes the problem of high background emission levels attendant with intermittently energized circuits, and allows them to be analyzed with background emission photometry.